1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device that includes a laser beam irradiation step, and the present invention also relates to a semiconductor device manufactured thereby. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light-emitting device, and an electronic device that includes the electro-optical device as a display portion.
2. Description of the Related Art
In recent years, an extensive study has been made on a technique in which an amorphous semiconductor film formed on an insulator, especially a glass substrate, is crystallized so that a crystalline semiconductor film is obtained. As the methods of crystallization such as a thermal annealing method using furnace annealing, a rapid thermal annealing method (RTA method), a laser annealing method and the like were examined. Anyone thereof or combining two or more methods thereof can be carried out for crystallization.
On the other hand, an active matrix display device using a thin film transistor (hereafter referred to as TFT) manufactured by forming a semiconductor thin film is widely adopted. Active matrix display devices using TFTs have several hundred thousand to several million pixels arranged in a matrix shape, and an image display is performed by controlling the electric charge of each pixel by using TFTs disposed in each pixel.
Recently, techniques that are used for simultaneously forming driver circuits by using TFTs in the periphery of a pixel portion in addition to pixel TFTs that comprise pixels have progressed. A crystalline semiconductor layer has extreme higher electric field effect mobility in comparison with an amorphous semiconductor layer, therefor, it is practicable for forming an active layer of TFT (hereafter simply referred to as active layer, the active layer includes a source region, a drain region, and a channel formation region) used for such circuits.
Generally, in order to crystallize an amorphous semiconductor in annealing furnace, a thermal treatment at 600° C. or more for 10 hours or more is required. Therefor, applicable material of substrates is limited to quartz that is capable of withstanding the heat treatment. However, the quartz substrate is expensive in price, and is difficult to be manufactured in a large area.
In order to improve the manufacturing efficiency, manufacturing the substrate in a large area, and mass production are unavoidable, it is expected that a substrate in which a length of one side exceeds 1 m will be also used in recent years.
On the other hand, a method of thermal crystallization by using metal elements disclosed in Japanese Patent Application Laid Open No. 7-183540 enable the crystallization temperature which was a conventional problem to be realized at a low temperature. The crystalline semiconductor film can be formed by this method in which a small amount of an element such as nickel, palladium and lead is added to an amorphous semiconductor film, then the amorphous semiconductor film is heated for four hours at 550° C.
Since the laser annealing method can deliver high energy only to the semiconductor film without substantially increasing the temperature in substrate by focusing the semiconductor layer, the laser annealing technology comes under spotlight by its appliance in a glass substrate with a low strain point as a matter of course, and a plastic substrate, etc.
An example of the laser annealing method is a method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on an irradiation surface, and relatively shifting an irradiation position of the laser beam with respect to the irradiation surface to conduct annealing. The “linear shape” described here means not a “line” in the strict sense but a rectangle or a prolate ellipsoid shape having a high aspect ratio. For example, the linear shape indicates a shape having an aspect ratio of 2 or more (preferably 10 to 100). Note that the linear shape is used to obtain an energy density required for annealing an object sufficiently to be irradiated. Thus, if sufficient annealing is conducted for the object to be irradiated, it may be a rectangular shape, a tabletop shape or any other shapes.
The state is shown in FIG. 8. After an amorphous semiconductor layer is formed on a substrate 801, a linear laser 803 is scanned and crystallized in a direction of an arrow. At this time, a cross-sectional view in which a dotted line is shown by A-A′ is shown in FIG. 8B. An insulation layer 811 is formed on the substrate 801 as a base layer, an amorphous semiconductor layer 813 is formed thereon. In addition, for the insulation layer 811, a single layer is illustrated in FIG. 8, but a structure without an insulation layer, or a structure of a laminate film having two or more layers also may be used.
Then, the linear laser 803 is scanned and irradiated on the substrate. At this time, in a region 812 irradiated by the linear laser, the amorphous semiconductor layer is in a state of molten, and after the passage of the irradiation region, the amorphous semiconductor layer is recrystallized. As described above, a crystalline semiconductor layer 815 is formed. However, a crystalline semiconductor film formed by subjecting an amorphous semiconductor film to laser annealing includes a collection of a plurality of crystal grains, and the position and size of the crystal grains are random. TFTs are formed on a glass substrate by patterning the crystalline semiconductor layer in an island shape for device separation. In this case, the position and size of crystal grains cannot be specified. In comparison with the inner of crystal grains, the interface of crystal grains has an infinite number of a recombination centers or a trapping centers caused by an amorphous structure, a crystal defect, and the like. If the carriers are trapped in trapping centers, potential at a grain boundary will be increased and become barriers to carriers, it is known that current transporting characteristics of carriers will be degraded caused by this. However, it is almost impossible to form a channel formation region by using a single crystal semiconductor film while avoiding the influence of a crystal boundary, although crystal properties of semiconductor film of channel formation region have a serious effect on the TFT characteristics.
There is a technique of irradiating a semiconductor layer with a CW (continuous wave) laser beam by running the beam in one direction to make a crystal grow continuously in the scanning direction and obtain a single crystal stretching long in the scanning direction. It is considered that this method can provide a TFT having almost no grain boundaries at least in its channel direction. However, in order to obtain excellent crystallinity, a region of an amorphous semiconductor layer that is irradiated with a laser has to be melted completely. For that reason, the laser irradiation region is converged into a rectangle or ellipse having a width of several hundreds μm to secure enough energy density and the surface of the irradiation object is scanned with laser light as shown in FIG. 1A to crystallize the entire surface thereof. As a result, crystal grains that are long in the scanning direction are formed to grow into a crystalline semiconductor layer as shown in FIG. 1B.
Here, attention is paid to the energy density in the laser irradiation width direction. When laser light is collected spot-like in a region, there is an energy density distribution starting from the center of the irradiation region toward the edges as in an example shown in FIG. 1C. Although the energy density distribution varies depending on the laser oscillation mode, a region low in energy density is generally incapable of providing an energy for melting a semiconductor layer sufficiently. This region of the semiconductor layer cannot grow large crystal grains and can only have microcrystals. Accordingly, in a semiconductor layer treated with a CW laser, a crystalline semiconductor layer A 112 where crystal grains grow into satisfactory large sizes and a crystalline semiconductor layer B 113 having microcrystals are formed for each scanned region (an irradiation region of when the CW laser scans one line) as shown in FIG. 1B.
In the semiconductor layer A, excellent electric characteristics are obtained as mentioned above. On the other hand, the semiconductor layer B has countless numbers of grain boundaries and therefore cannot provide satisfactory electric characteristics.
If the semiconductor layer as such is patterned and used to form TFTs, there is a large difference in electric characteristic between a TFT which includes the semiconductor layer B in its channel formation region and a TFT which doesn't. Therefore, it is difficult to manufacture a semiconductor device that operates satisfactorily from these TFTs despite many elements included in them which have excellent electric characteristics.